Fix coverity defects found on the FPGA port.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I397b642eff8a09b201f497f8d2ba39e2460c0dba

plat/arm/board/arm_fpga/fpga_pm.c

@@ -41,9 +41,13 @@ uintptr_t fpga_sec_entrypoint;
  */
 static int fpga_pwr_domain_on(u_register_t mpidr)
 {
-	unsigned int pos = plat_core_pos_by_mpidr(mpidr);
+	int pos = plat_core_pos_by_mpidr(mpidr);
 	unsigned long current_mpidr = read_mpidr_el1();
 
+	if (pos < 0) {
+		panic();
+	}
+
 	if (mpidr == current_mpidr) {
 		return PSCI_E_ALREADY_ON;
 	}